A conventional water processing system designed for processing highly contaminated water typically comprises an anaerobic processing unit which is suitable for processing highly contaminated water and an aerobic processing unit, such as the standard activated sludge process, which is capable of producing relatively pure water. In such a processing system combining an anaerobic processing unit and an aerobic processing unit, a part of the water processed by the aerobic processing unit is in some cases recycled to the anaerobic processing unit so that the nitrogen content nitrified in the aerobic processing unit is denitrified in the anaerobic processing unit, and the nitrogen content which is known to undesirably eutrophicate rivers and lakes can be effectively removed.
However, when the processing load is heavy, it is necessary to provide additional facilities for carrying out such preliminary processes as aggregation/precipitation and floatation separation processes. As a result, operation of the water processing system becomes unacceptably complex, and the overall system is made undesirable large in size. Furthermore, the space ratio of the contact material is required to be as high as 98% to prevent clogging of the processing vessel in both the anaerobic and aerobic processing units, and this prevents the attainment of a high level of purification, the typical final BOD and T-N (total nitrogen content) being no less than 20 mg/liter and 10 mg/liter, respectively.
Also, it is difficult to maintain nitrifying microbes, for instance, in a standard activated sludge processing tank for promoting the nitrification of the nitrogen content in the aerobic processing unit, and it is therefore necessary to carry out extensive aeration for satisfactory nitrification and denitrification to take place. As a result, a relatively long resident time becomes necessary. In the water processing systems which employ such carriers as plastic frames and honeycomb structures, the space ratio is so high that the ratio of nitrification typically ranges between 30 and 40%. The resident time has to be increased to achieve a higher ratio of nitrification which, however, would not be any more than 50%. Thus, according to the conventional technology, a high nitrification ratio cannot be achieved even when the resident time is increased, and the size of the system is increased.